Systems and methods for communicating vectoring streams in multi-card vectoring groups

ABSTRACT

A communication system comprises a plurality of line cards having transceivers coupled to a plurality of subscriber lines. Each line card has at least one active transceiver within the same vectoring group, and each line card also has vector logic capable of cancelling crosstalk induced by an active transceiver that is a member of the vectoring group. Further, the line cards are coupled to one another via a ring connection across which vectoring information is passed from one line card to the next. In the event of a failure of one of the line cards, the failed card is bypassed by the vectoring stream so that the operational line cards can continue crosstalk vectoring operations despite such failure.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. patent application Ser. No.61/448,546, entitled “Generalized Vectoring Architecture” and filed onMar. 2, 2011, which is incorporated herein by reference.

RELATED ART

A digital subscriber line access multiplexer (DSLAM) is a device thatconnects multiple subscriber lines to a high-speed network line usingdigital subscriber line (DSL) modulation formats across the subscriberlines. In the downstream direction, a DSLAM generally demultiplexes ahigh-speed data stream from a network across the subscriber lines, andin the upstream direction, a DSLAM generally multiplexes the datastreams from the subscriber lines for transmission across the high-speednetwork line. A DSLAM can be installed at a variety of locations, suchas at a network facility (e.g., a central office) or an intermediatepoint between a central office and one or more customer premises.

A variety of DSL formats have been used for the communication from aDSLAM to a customer premises. Very-high-bit-rate DSL (VDSL) is asolution that is attractive due to the relatively high data ratesenabled by VDSL as compared to other DSL solutions. Indeed, firstgeneration VDSL provides data transmission up to about 52 Mega-bits persecond (Mbit/s) downstream and about 16 Mbit/s upstream. Secondgeneration VDSL, sometimes referred to as VDSL2, provides up to about100 Mbit/s simultaneously in the both the upstream and downstreamdirections. The VDSL line code is discrete multi-tone (DMT) modulation,which uses a sequence of equally spaced frequencies or tones, each ofwhich is modulated using quadrature amplitude modulation (QAM).

Like several other DSL technologies, VDSL suffers from the effects ofcrosstalk. However, VDSL standards specify vectoring techniques thatallow crosstalk cancellation, and such techniques have been employed tocancel the crosstalk among subscriber lines extending from a DSLAM toone or more customer premises in an effort to improve the performance ofVDSL signals and allow for longer reaches. However, VDSL vectoring isprocessing intensive, and as the number of subscriber lines increases,the amount of processing required to cancel crosstalk from the signalscarried by the subscriber lines increases exponentially.

In this regard, to cancel crosstalk, vector logic (sometimes referred toas a “vector engine”) maintains a set of vectoring coefficientsrespectively corresponding to the coupling functions between interferingtones (i.e., tones that induce crosstalk) and victim tones (i.e., tonesaffected by crosstalk). For each victim tone, the vector engine combinesthe symbol of the victim tone and each interfering tone with itscorresponding vectoring coefficient, and the result becomes the newvictim tone. In such manner, the crosstalk channel is effectivelyinverted, and the crosstalk is cancelled tone-by-tone for all tones. Fortransmitter-based precoding, the new victim tone is transmitted on theline, and the precoding effectively cancels the crosstalk as the victimtone propagates across the line and the crosstalk couples into the lineof the victim tone. For receiver-based cancellation, the new tone isfurther processed by the receiver but with reduced crosstalk.

A single vector engine can process the coefficients for a limited numberof tones, but additional vector engines can be added in order toincrease the number of tones subject to the VDSL vectoring. In suchcase, the vector engines share vectoring information (e.g., tonesymbols) so that a larger number of interfering tones can be canceledfrom each victim tone.

When a vectoring group spans across multiple line cards, the vectoringinformation should be communicated among the line cards. Reliablycommunicating vectoring information among multiple line cards can beproblematic, particularly for vectoring groups that process a largenumber of tones thereby generating a large amount of vectoringinformation.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure can be better understood with reference to the followingdrawings. The elements of the drawings are not necessarily to scalerelative to each other, emphasis instead being placed upon clearlyillustrating the principles of the disclosure. Furthermore, likereference numerals designate corresponding parts throughout the severalviews.

FIG. 1 is a block diagram illustrating an exemplary embodiment of acommunication system.

FIG. 2 is a block diagram illustrating an exemplary embodiment of anetwork access point, such as is depicted by FIG. 1.

FIG. 3 is a block diagram illustrating an exemplary embodiment of arelay element, such as is depicted by FIG. 2, while the relay element isin a pass-through state.

FIG. 4 is a block diagram illustrating an exemplary embodiment of arelay element, such as is depicted by FIG. 3, while the relay element isin a bypass state.

FIG. 5 is a block diagram illustrating an exemplary vectoring streamcommunicated across a ring connection, such as is depicted by FIG. 2.

FIG. 6 is a flowchart illustrating an exemplary method of processing avectoring stream, such as is depicted by FIG. 5.

FIG. 7 is a block diagram illustrating an exemplary embodiment of agroup of line cards implemented at a network access point, such as isdepicted by FIG. 1.

FIG. 8 is a block diagram illustrating an exemplary embodiment of a linecard, such as is depicted by FIG. 7.

FIG. 9 is a block diagram illustrating an exemplary flow of a vectoringstream for a plurality of line cards, such as is depicted by FIG. 7.

FIG. 10 is a block diagram illustrating an exemplary flow of a vectoringstream for a plurality of line cards, such as is depicted by FIG. 9,after one of the line cards has failed.

FIG. 11 is a flowchart illustrating an exemplary method of processing avectoring stream.

DETAILED DESCRIPTION

The present disclosure generally pertains to systems and methods forcommunicating vectoring information for multi-card vectoring groups. Inone exemplary embodiment, a communication system comprises a pluralityof line cards having transceivers coupled to a plurality of subscriberlines. Each line card has at least one active transceiver within thevectoring group, and each line card also has vector logic capable ofcancelling crosstalk induced by an active transceiver that is a memberof the vectoring group. Further, the line cards are coupled to oneanother via a ring connection across which vectoring information ispassed from one line card to the next. In this regard, the ringconnection carries a data stream, referred to hereafter as “vectoringstream,” having a plurality of time slots respectively allocated to theline cards. As the vectoring stream is communicated, each line cardinserts vectoring information (e.g., the symbols received by such linecard from the subscriber lines or to be transmitted by such line cardacross the subscriber lines) into the time slots allocated to it.Further, each line card reads the vectoring information of other linecards from the time slots allocated to the other line cards.Accordingly, the vector logic on a given line card has sufficient accessto the vectoring information of the other line cards to cancel crosstalkinduced by interfering tones communicated across any of the subscriberlines regardless of which line cards receive or transmit the interferingtones. In the event of a failure of one of the line cards, the failedcard is bypassed by the vectoring stream so that the operational linecards can continue crosstalk vectoring operations despite such failure.

FIG. 1 depicts an exemplary embodiment of a communication system 10. Asshown by FIG. 1, the system 10 comprises a network 12, such as thepublic switched telephone network (PSTN) or other communication network,configured to communicate with CPE 15 at one or more customer premises21. In this regard, the network 12 is coupled to a network access point25 via a network line 27, such as one or more twisted-wire pairs oroptical fibers, and the network access point 25 is coupled to the CPE 15via a plurality of subscriber lines 29, such as twisted-wire pairs oroptical fibers. For simplicity, FIG. 1 depicts four subscriber lines 29,but there can be any number of subscriber lines 29 in other embodiments.Further, the lines 29 in FIG. 3 extend to multiple customer premises 21,but it is also possible for the lines 29 to extend to a single customerpremises 21.

In a downstream direction, the network access point 25 receives ahigh-speed data stream from the network 12 via the network line 27 andforwards packets from the high-speed data stream across the plurality ofsubscriber lines 29. In an upstream direction, the network access point25 receives data streams from the customer premises 21 via thesubscriber lines 29 and transmits packets from such data streams acrossthe network line 27 to the network 12.

In one exemplary embodiment, the network line 27 comprises an opticalfiber, and optical modulation formats are used to communicate dataacross the fiber. In addition, each subscriber line 29 comprises atleast one twisted-wire pair, and digital subscriber line (DSL)modulation formats are used to communicate data across the subscriberlines 29. Note that there are a variety of DSL modulation formats thatmay be used for communicating data across the subscriber lines 29, suchas asymmetric DSL (ADSL), high-bit-rate DSL (HDSL), very-high-bit-rateDSL (VDSL), and single-pair HDSL (SHDSL). For illustrative purposes, itwill be assumed hereafter that the modulation format used for eachsubscriber line is VDSL, such as first generation VDSL or VDSL2, but itshould be emphasized that other DSL and/or non-DSL modulation formatsmay be used in other embodiments.

FIG. 2 depicts an exemplary embodiment of the network access point 25.The network access point 25 comprises a plurality of line cards 51-53coupled to the subscriber lines 29. In this regard, each line card 51-53has at least one transceiver 55 coupled to at least one subscriber line29. In the exemplary embodiment shown by FIG. 2, each line card 51-53has three transceivers 55 respectively coupled to three subscriber lines29, but any line card 51-53 may have any number of transceivers 55and/or be coupled to any number of subscriber lines 29 in otherembodiments. Further, it is unnecessary for each line card 51-53 to havethe same number of transceivers 55 and be coupled to the same number ofsubscriber lines 29 as the other line cards. As will be described inmore detail hereafter, each transceiver 55 is a member of the samevectoring group, but it is unnecessary for all of the transceivers 55 tobe members of the same vectoring group in other embodiments. In oneexemplary embodiment, the components of the same line card reside on asingle printed circuit board (PCB), but it is possible for any line cardto have more than one PCB, if desired.

As shown by FIG. 2, each line card 51-53 has a card control element 63that is coupled to each transceiver 55 residing on the same line card51-53. The card control element 63 controls the general operation of theline card 51-53 on which it resides, including the forwarding of datapackets, as will be described in more detail hereafter. The card controlelement 63 may be implemented in hardware, software, firmware, or anycombination thereof. Each card control element 63 is also coupled to aport 66, referred to hereafter as a “network-side port,” which iscoupled to a switching element 71. Further, the switching element 71 iscoupled to a network transceiver 74 that is coupled to the network line27.

In the downstream direction, assuming that the network line 27 comprisesan optical fiber, the network transceiver 74 receives an optical datasignal from the network line 27 and converts the optical data signalinto an electrical signal comprising data packets. The switching element71 is configured to forward the data packets to the line cards 51-53based on header information within the data packets using forwardingtechniques known in the art. Each data packet received by a line card51-53 is received by the card's control element 63, which forwards thepacket to at least one transceiver 55 for transmission across at leastone subscriber line 29. When a transceiver 55 receives data packets fromthe card control element 63, the transceiver 55 modulates a carriersignal with the data packets using VDSL or some other desired modulationformat in order to form a data signal that is transmitted across thesubscriber line 29 coupled to such transceiver 55.

In the upstream direction, the transceivers 55 receive modulated datasignals from the subscriber lines 29. For a given signal received by agiven transceiver 55, the transceiver 55 demodulates the received signalto recover data packets. The transceiver 55 forwards the data packets tothe card control element 63 that is residing on the same line card51-53, and the card control element 63 forwards each packet receivedfrom the card's transceivers 55 to the switching element 71. Theswitching element 71 combines the packets received from all of the linecards 51-53 into a high-speed data stream, which is received by thenetwork transceiver 74. Assuming that the network line 27 comprises anoptical fiber, the network transceiver 74 converts the received datastream into an optical data signal for transmission across the networkline 27 to the network 12 (FIG. 1).

As shown by FIG. 2, each line card 51-53 comprises vector logic 77(e.g., one or more vector engines) configured to perform crosstalkvectoring in order to cancel crosstalk from the tones transmitted and/orreceived by the card 51-53 on which it resides. For example, the vectorlogic 77 of the line card 51 maintains vectoring coefficients for thetones communicated (i.e., transmitted or received) by the transceivers55 of the card 51. Via known techniques, the vector logic 77 uses suchvectoring coefficients to estimate crosstalk that affects the receivedtones or that will affect tones to be transmitted so that such crosstalkcan be cancelled.

For example, the vector logic 77 of the card 51 stores sets of vectoringcoefficients respectively corresponding to the tones communicated acrossthe subscriber lines 29 coupled to the card 51. For a given tone,referred to in this example as “victim tone,” received by one of thetransceivers 55 from one of the subscriber lines 29, the correspondingset of vectoring coefficients includes vectoring coefficientsrespectively associated with the tones, referred to as “interferingtones,” that interfere with the victim tone. Upon receiving a set ofsymbols simultaneously communicated across the subscriber lines 29, thetransceivers 55 of the card 51 send such symbols to the vector logic 77of the card 51. For each interfering tone, the vector logic 77 combines(e.g., multiplies) the tone's symbol with the associated vectoringcoefficient to estimate an amount of crosstalk interference from theinterfering tone affecting the victim tone. The vector logic 77 thencancels such crosstalk interference from the symbol of the victim toneby combining (e.g., subtracting) the estimate with the symbol of thevictim tone. The vector logic 77 performs the same process for thesymbols of the other interfering tones in order to cancel, tone-by-tone,crosstalk interference in the victim tone.

After removing the crosstalk interference induced by the interferingtones, the vector logic 77 sends the symbol of the victim tone back tothe transceiver 55 that originally received it from a subscriber line29. Such transceiver 55 decodes the symbol and generates an error signalindicating an error estimate for the symbol. The foregoing transceiver55 sends the error signal to the vector logic 77, which adaptivelyupdates the set of vectoring coefficients corresponding to the victimtone based on the error signal using a known coefficient updatealgorithm, such as least means square (LMS).

Thus, the vector logic 77 of the card 51 cancels, from the symbol of thevictim tone, crosstalk induced by each interfering tone received by thecard 51 from the subscriber lines 29. In one exemplary embodiment, thevector logic 77 uses the same techniques to cancel, from the symbol ofthe victim tone, crosstalk induced by interfering tones received by theother line cards 52 and 53. In this regard, the set of vectoringcoefficients corresponding to the victim tone also includes vectoringcoefficients associated with the tones received by the cards 52 and 53.As described above for the symbols received by the card 51, the vectorlogic 77 combines the symbols received by the transceivers 55 of theline cards 52 and 53 with the associated vectoring coefficients toestimate the amount of crosstalk induced by such symbols in the symbolof the victim tone. The vector logic 77 also combines such estimateswith the symbol of the victim tone to cancel, tone-by-tone, thecrosstalk induced by the interfering tones received by the line cards 52and 53.

Further, such vector logic 77 uses similar techniques to precode thesymbols transmitted by the transceivers 55 of the card 51 across thesubscriber lines 29 such that crosstalk is cancelled as the symbolspropagate across the subscriber lines 29. Specifically, the vector logic77 receives symbols of interfering tones to be communicated across thelines 29 at the same time as a symbol of a victim tone to becommunicated by the line card 51. For each interfering tone, the vectorlogic 77 combines (e.g., multiplies) the tone's symbol with theassociated vectoring coefficient to estimate an amount of crosstalkinterference that will affect the symbol of the victim tone. The vectorlogic 77 then combines (e.g., subtracts) the estimate from the symbol ofthe victim tone in order to precode the symbol so that crosstalkinterference from the interfering tone is effectively cancelled duringcommunication as it couples into the line 29 of the victim tone. Thevectoring coefficients used for the precoding are updated based on theerror associated with the victim tone, as determined by the CPtransceiver 15 (FIG. 1) that receives the victim tone. Exemplarytechniques for performing crosstalk vectoring, including symbolprecoding, are described in commonly-assigned U.S. patent applicationSer. No. 13/016,680, entitled “Systems and Methods for CancellingCrosstalk in Satellite Access Devices” and filed on Jan. 28, 2011, whichis incorporated herein by references. Note that the vector logics 77 ofthe other line cards 52 and 53 are configured similar to the vectorlogic 77 of the line card 51 in order to cancel crosstalk affecting thetones communicated by the lines cards 52 and 53, respectively.

Moreover, in one exemplary embodiment, the vector logic 77 andtransceivers 55 of the line cards 51-53 form a vectoring group such thatcrosstalk from any interfering tone communicated across the subscriberlines 29 can be cancelled from any victim tone communicated across thesubscriber lines 29. However, in other embodiments, it is unnecessaryfor each vector logic 77 and each transceiver 55 at the network accesspoint 25 to be a member of the same vectoring group.

In order to enable each line card 51-53 to cancel crosstalk induced bytones communicated by the other line cards, vectoring information (e.g.,tone symbols) is passed from one card to the other. For example, toenable the vector logic 77 of the line card 51 to cancel, from a victimtone received by the line card 51, crosstalk induced by interferingtones received by the line card 53, the line card 53 transmits symbolsof the interfering tones to the line card 51. Similarly, to enable thevector logic 77 of the line card 51 to precode the symbols transmittedby the transceivers 55 of the line card 51 such that crosstalk inducedby interfering tones transmit by line card 53 is cancelled as thesymbols propagate across the subscriber lines 29, the line card 53transmits symbols of the interfering tones to the line card 51. In oneexemplary, a ring connection 80 comprising a plurality of segments 81-83is configured to carry vectoring information from card-to-card. In thisregard, each line card 51-53 has a plurality of ports 87 and 88,referred to hereafter as “ring ports,” respectively coupled to aplurality of segments of the ring connection 80. For example, the ringport 87 of the line card 51 is coupled to one end of the segment 83, andthe other end of the segment 83 is coupled to the ring port 88 of theline card 52. Further, the ring port 87 of the line card 52 is coupledto one end of the segment 82, and the other end of the segment 82 iscoupled to the ring port 88 of the line card 53. Also, the ring port 87of the line card 53 is coupled to one end of the segment 81, and theother end of the segment 81 is coupled to the ring port 88 of the linecard 51.

As shown by FIG. 2, each ring port 87 and 88 of a given line card 51-53is coupled to a relay element 91, and the relay element 91 is coupled toand controlled by a ring interface 94, which is coupled to the card'svector logic 77. The ring interface 94 may be implemented in hardware,software, firmware, or any combination thereof.

The relay element 91 of a given line card 51-53 is configured totransition between a state, referred to herein as the “bypass state,”and a state, referred to herein as the “pass-through state.” In thebypass state, the relay element 91 is configured to short the ring ports87 and 88 such that the data carried by the ring connection 80effectively bypasses the logic on the card 51-53 on which the relayelement 91 resides. That is, the relay element 91 electrically couplesthe ring port 88 directly to the ring port 87. Thus, when the relayelement 91 of a given card 51-53 is in the bypass state, the datareceived from one ring port 87 or 88 is passed directly to the otherring port of the same card thereby bypassing the logic (e.g., ringinterface 94 and vector logic 77) of the same card.

In the pass-through state, the relay element 91 electrically coupleseach ring port 87 and 88 to the ring interface 94. Thus, the vectoringinformation carried by the ring connection 80 is received and processedby the ring interface 94. Ultimately, such information is used by thevector logic 77 to perform crosstalk vectoring operations.

FIG. 3 depicts an exemplary embodiment of the relay element 91, which isshown to be in the pass-through state. The exemplary relay element 91has a plurality of switches 97-99, which will be described in moredetail below. Each switch 97-99 is configured to transition between twostates, referred to herein as the “open state” and the “closed state.”When in the open state, the switch 97-99 operates as an open circuit bypreventing current from flowing. When in the closed state, the switch97-99 operates as a short circuit by allowing current to flow.

When the switch 97 is in the closed state, the switch 97 electricallycouples node 101 to node 102. In such state, the switch 97 provides aconductive path for the data carried by the ring connection 80 (FIG. 2)such that the data bypasses the ring interface 94 and, hence, the otherlogic residing on the same line card, such as the vector logic 77 (FIG.2). However, in FIG. 3, the switch 97 is shown in the open state suchthat the relay element 91 does not provide a conductive path directlybetween the nodes 101 and 102.

In FIG. 3, the switch 98 is in the closed state and, therefore,electrically couples the node 102 to a node 103, thereby providing aconductive path from the ring port 88 to the ring interface 94. Further,the switch 99 is also in the closed state and, therefore, electricallycouples the node 101 to a node 104, thereby providing a conductive pathfrom the ring port 87 to the ring interface 94. Accordingly, when therelay element 91 is in the pass-through state, as shown by FIG. 3, thedata carried by the ring connection 80 (FIG. 2) passes through the relayelement 91 to the ring interface 94.

FIG. 4 depicts an exemplary embodiment of the relay element 91 after ithas been transitioned to the bypass state. In FIG. 4, the switches 98and 99 are both in the open state such that the node 102 is electricallyisolated from the node 103 and such that the node 101 is electricallyisolated from the node 103. Thus, the data carried by the ringconnection 80 (FIG. 2) is prevented from passing through the relayelement 91 to the ring interface 94. Further, as shown by FIG. 4, theswitch 97 is in the closed state such that the node 101 is electricallycoupled to the node 102. Accordingly, when the relay element 91 is inthe bypass state, as shown by FIG. 4, the data carried by the ringconnection 80 (FIG. 2) passes through the relay element 91 from one ringport 87 or 88 to the other such that the data bypasses the ringinterface 94 and, hence, the other logic residing on the same card, suchas the vector logic 77 (FIG. 2). For example, if data is received by thering port 88, such data flows through the closed switch 97 to the ringport 87, and if data is received by the ring port 87, such data flowsthrough the closed switch 97 to the ring port 88.

In one exemplary embodiment, the vectoring stream carried by the ringconnection 80 (FIG. 2) has time slots that are respectively allocated tothe line cards 51-53, and each line card 51-53 inserts into the slotsallocated to it vectoring information for enabling the other line cardsto cancel crosstalk. As an example, refer to FIG. 5, which depicts anexemplary embodiment of a vectoring stream 125 carried by the ringconnection 80. In this regard, the vectoring stream 125 comprises afield 131 allocated to the line card 51, a field 132 allocated to theline card 52, and a field 133 allocated to the line card 53. Each linecard 51-53 is configured to insert vectoring information into the slotsof the respective field 131-133 allocated to it, and the fields 131-133shall be referred to herein as “vectoring fields.”

For example, the vector logic 77 of the line card 51 is configured totransmit to the card's ring interface 77 the symbols of the tonescommunicated across the subscriber lines 29 coupled to the card 51. Thering interface 94 is configured to insert such symbols into the slots ofthe vectoring field 131 allocated to the card 51. The vector logics 77of the other cards 52 and 53 are configured to use such symbols tocancel crosstalk. As an example, the vector logic 77 of the line card 52may use the symbols in the vectoring field 131 to cancel crosstalkinduced by the interfering tones received by the line card 51 from thesubscriber lines 29 coupled to such card 51. Further, the vector logicof the line card 52 may use the symbols in the vectoring field 131 toprecode the symbols transmitted by the transceivers 55 of the line card52 such that crosstalk induced by interfering tones transmitted by theline card 51 is cancelled as the symbols propagate across the subscriberlines 29. Similarly, the other cards 52 and 53 are configured to insertsymbols into the vectoring fields 132 and 133, respectively, allocatedto such cards 52 and 53. Thus, the data carried by the ring connection80 provides the symbols communicated across all of the subscriber lines29 associated with the vectoring group so that each card 51-53 cancancel crosstalk induced by an interfering tone communicated by anysubscriber line 29, regardless of which card 51-53 actually transmits orreceives the interfering tone via the subscriber lines 29.

As shown by FIG. 5, the vectoring stream 125 also comprises a field 134,referred to hereafter as “control field,” that may be used by the linecards 51-53 to transmit control information among the cards 51-53, aswill be described in more detail hereafter. In one exemplary embodiment,the vectoring stream 125 is transmitted across the ring connection 80(FIG. 2) from card-to-card in the same direction around the ring formedby the ring connection 80 and cards 51-53. For example, referring toFIG. 2, the vectoring stream 125 may be transmitted from the line card51 to the line card 52 via the segment 83, from the line card 52 to theline card 53 via the segment 82, and from the line card 53 to the linecard 51 via the segment 81. Alternatively, the vectoring stream 125 maybe transmitted from the line card 51 to the line card 53 via the segment81, from the line card 53 to the line card 52 via the segment 82, andfrom the line card 52 to the line card 51 via the segment 83. In otherembodiments, other techniques for communicating the vectoring stream 125are possible. For example, as will be described in more detailhereafter, it is possible for the vectoring stream 125 to be transmittedin both directions, or for one portion of the vectoring stream 125 to betransmitted in one direction while another portion of the vectoringstream 125 is transmitted in the opposite direction. For illustrativepurposes, it will be assumed hereafter that the vectoring stream 125 istransmitted in the same direction, clockwise, around the ring unlessotherwise noted. Thus, unless otherwise noted, it will be assumed thatthe vectoring stream 125 propagates from the segment 81 to the segment83 and then to the segment 82.

During operation, it is possible for a failure to occur on any line card51-53. Depending on the type of failure that occurs, it is also possiblethat the failure may prevent the line card from being able tocommunicate the vectoring stream 125. However, in one exemplaryembodiment, the continued transmission of the vectoring stream 125despite a failure of any line card 51-53 is ensured via the relayelements 91. In this regard, the relay elements 91 are normally in thepass-through state. However, when a line card 51-53 fails such that itcan no longer receive and/or transmit the vectoring stream 125 via thering interface 94, the relay element 91 of the failed card istransitioned to the bypass state such that the vectoring stream 125bypasses the failed line card. That is, for the failed line card, thevectoring stream 125 passes from the ring port 88 to the ring port 87through the relay element 91 (assuming a clockwise data flow) withoutbeing received and/or processed by the ring interface 94. Thus, failureof the line card does not prevent successful communication of thevectoring stream 125 among the other line cards.

In one exemplary embodiment, the ring interface 94 of each line card51-53 is configured to monitor the status of the line card on which itresides and to detect a failure of such card that prevents the ringinterface 94 from successfully communicating the vectoring stream 125.In response to the detection of such a failure, the ring interface 94 isconfigured to control the relay element 91 such that it transitions fromthe pass-through state to the bypass state.

However, for some failures, the ring interface 94 may be prevented fromcontrolling the state of the relay element 94. As an example, in oneexemplary embodiment, each line card 51-53 has a power bus (notspecifically shown) that carries electrical power to the circuitry ofthe line card, such as the transceivers 55, the control element 63, thevector logic 77, the ring interface 94, and the relay element 91. Due toa card failure, the ring interface 94 and/or the relay element 91 may beprevented from receiving power from the power bus. In such case, thering interface 94 may be unable to detect the failure or to change thestate of the relay element 91. In one exemplary embodiment, the relayelement 91 is configured to automatically transition to the bypass statewhen it stops receiving electrical power from the power bus. Thus, therelay element 91 is transitioned to the bypass state even if the ringinterface 94 is unable to control the state of the relay element 91. Inother embodiments, other techniques for controlling the state of thering element 91 are possible.

In one exemplary embodiment, when a line card 51-53 fails andtransitions its relay element 91 to the bypass state, the other linecards discover such event and update the vectoring performed by suchline cards in order to remove the tones associated with the failed linecard from the crosstalk vectoring. For example, assume that the linecard 51 communicates an interfering tone that interferes with a victimtone received by the line card 52. Further assume that the line card 51fails causing the relay element 91 of the line card 51 to transition tothe bypass state and preventing communication of the interfering toneacross the subscriber lines 29. In such example, if the vector logic 77of the line card 52 attempts to perform crosstalk vectoring to cancelthe effects of the interfering tone from the victim tone based on thevectoring information in the vectoring stream 125, it is likely that theattempt to cancel the effects of the interfering tone will introducenoise into the victim tone, thereby degrading the quality of the victimtone. To prevent such an effect, the line card 52 is configured todetect the failure of the line card 51, and the vector logic 77 of theline card 52 is configured to disable crosstalk vectoring for theinterfering tone communicated by the failed card 51. In such case, thevector logic 77 of the line card 52 is configured to perform crosstalkvectoring only for interfering tones communicated by the operationalcards 52 and 53.

There are various techniques that can be used to enable one line card51-53 to detect the failure of another. Various exemplary techniques fordetecting a failed line card will be described in more detail below, butit should be emphasized that other techniques are possible in otherembodiments.

In one exemplary embodiment, when a line card 51-53 fails, the vectoringfield 131-133 allocated to the failed card is zeroed such that all ofthe data values in the slots of such field have a value of 0. Thus, thepresence of all zeros in a vectoring field 131-133 indicates that theline card to which the field is allocated has failed. Further, if avector logic 77 of any line card attempts to perform crosstalk vectoringbased on the vectoring field of the failed card, multiplication of azero value with any vectoring coefficient is equal to zero, andsubtracting such a zero value from any symbol during a vectoringoperation should not affect such symbol. Thus, the presence of the zerosin the vectoring field 131-133 of the failed card effectively disablescrosstalk vectoring for the interfering tones associated with the failedcard.

There are various techniques that can be used to zero the vectoringfield 131-133 of the failed line card 51-53. In one exemplaryembodiment, each line card 51-53 is configured to write zeros into allof the slots of the vectoring field 131-133 allocated to the next linecard that is to receive the vectoring stream 125. Thus, when thevectoring stream 125 arrives at a given line card 51-53, the vectoringfield 131-133 allocated to such card should be zeroed. If such line cardis operational, the card's ring interface 94 overwrites the zeros withvectoring information from the card's vector logic 77. If such line cardhas failed such that its relay element 91 is in the bypass state, thenthe zeros are not overwritten.

As an example, as described above, the line card 51 is configured toinsert into the field 131 vectoring information comprising symbols oftones communicated by the transceivers 55 of the card 51. As thevectoring stream 125 propagates along the ring connection 80, the ringinterface 94 of the card 52 receives the vectoring field 131 and passesthe vectoring information from such field to the card's vector logic 77,which uses such information in performing vectoring operations. Sincethe line card 53 that is to receive the vectoring field 131 next is notthe one to which the field 131 is allocated, the ring interface 94 ofthe line card 52 does not overwrite the vectoring field 131 beforetransmitting it to the line card 53. The ring interface 94 of the linecard 53, like the ring interface 94 of the line card 52, passes theinformation in the vectoring field 131 to its respective vector logic77, which performs crosstalk vectoring based on such information.However, since the line card 51 that is to receive the vectoring field131 next is the one to which the field 131 is allocated, the ringinterface 94 of the line card 53 overwrites the vectoring information inthe field 131 with all zeros. Note that the ring interface 94 of theline card 53 does not overwrite the vectoring field 132 allocated to theline card 52, and such ring interface 94 overwrites the vectoring field133 allocated to the line card 53 with new vectoring information (e.g.,new symbols received and/or to be transmitted by the transceivers 55 ofthe line card 53).

When the line card 51 receives the vectoring field 131 allocated to it,the time slots of the vectoring field 131 have been zeroed by theprevious line card 53 from which it is received. If the line card 51 isoperational, the card's ring interface 94 overwrites the vectoring field131 with new vectoring information (e.g., new symbols received and/or tobe transmitted by the transceivers 55 of the line card 51). However, ifthe line card 51 has failed such that its relay element 91 is in thebypass state, then the line card 51 does not overwrite the vectoringfield 131. In such case, the vectoring field 131 received by line cards52 and 53 is zeroed. The presence of the zeros in the field 131indicates to the line cards 52 and 53 that the line card 51 has failed,and the vector logics 77 of the line cards 52 and 53 while performingcrosstalk vectoring do not change any symbols based on the vectoringfield 131.

In another exemplary embodiment, the control field 134 of the vectoringstream 125 is used to indicate when a line card 51-53 has failed. As anexample, the control field 134 may have control bits respectivelyassociated with the line cards 51-53. Further, the ring interface 94 ofeach line card 51-53 is configured to toggle the control bit associatedwith the line card each time the control field 134 is received by thering interface 94. Thus, the other line cards can determine whether agiven line card has failed based on whether the card's associated bithas been toggled since the last time that the control field 134 wastransmitted around the ring.

As an example, assume that the line card 51 is operational such that itsrelay element 91 is in the pass-through state. When the ring interface94 of the line card 51 receives the control field 134, the ringinterface 94 toggles the control bit associated with the line card 51.That is, the ring interface 94 transitions the control bit to a logical1 if the control bit is a logical 0 at reception, and the ring interface94 transitions the control bit to a logical 0 if the control bit is alogical 1 at reception. For illustrative purposes, assume thatassociated control bit was a logical 0 the last time that the controlfield 134 was transmitted around the ring. Upon receiving the controlfield 134, the ring interface 94 of the line card 51 toggles theassociated control bit to a logical 1. Thus, when the control field 134is received by the line cards 52 and 53, the ring interfaces 94 of theline cards 52 and 53 determine that the control bit associated with theline card 51 has been toggled to a new value indicating that the linecard 51 is operational. In such case, the line cards 52 and 53 use thevectoring field 131 allocated to the line card 51 to perform crosstalkvectoring, as described herein.

However, assume that the line card 51 fails just prior to receiving thecontrol field 134 in which the associated bit is a logical 0. In suchcase, the relay element 91 is in the bypass state when the control field134 arrives at the line card 51, and the ring interface 94 of the linecard 51 does not toggle the associated control bit, which remains alogical zero value. Thus, when the line card 52 receives the controlfield 134, the ring interface 94 of the line card 52 determines that theline card 51 has failed since the associated control bit has not beentoggled. In response to such determination, the ring interface 94 of theline card 52 overwrites the vectoring field 131 allocated to the failedcard 51 with zeros. In such case, neither line card 52 nor 53 inperforming crosstalk vectoring changes their respective tone symbolsbased on the vectoring field 131 allocated to the failed card 51.

Note that it is unnecessary for the vectoring field 131 allocated to thefailed card 52 to be overwritten with zeros. As an example, eachoperational line card 52 and 53 may be configured to disable vectoringoperations based on the vectoring field 131 in response to adetermination that the line card 51 has failed without actually changingthe values in the vectoring field 131 and/or changing the vectoringfield 131 in other ways.

In addition, it should be noted that the effective length of the ringconnection 80 between operational cards may be increased due to atransition of a relay element 91 to the bypass state. As an example, ifthe line card 51 fails, then the data transmitted across the segment 81to the line card 51 bypasses the circuitry of the line card 51 and isreceived by the line card 52. Thus, the transmission distance of thedata output via the ring port 87 of the line card 53 is effectivelyincreased from approximately the length of the segment 81 toapproximately the combined length of the segments 81 and 83. Thisadditional length increases attenuation and delay. In designing thetransmitters and receivers for the ring connection 80, theircharacteristics, such as transmit power, receive sensitivity, andtiming, should be designed to accommodate the effective transmissiondistances that result when a given relay element 91 transitions from thepass-through state to the bypass state.

Once a line card failure is remedied, such as for example replacing thefailed line card, the line cards 51-53 preferably resume normaloperation and, thus, communicate across the ring connection 80 andperform vectoring as described above prior to the card failure. In theembodiment described above in which the line card preceding the failedline card overwrites the vectoring field of the failed line card withzeros, the new line card (which replaces the failed line card) maysimply begin writing its symbols into the vectoring field allocated toit. The presence of non-zero values in such field indicates to the otherline cards that the previous card failure has been remedied. In otherembodiments, the new line card communicates control information via thecontrol field 134 to indicate that the previous failure has beenremedied and normal operation should resume. Various other techniquesfor resuming normal operation after remedying a card failure arepossible.

An exemplary operation and use of the ring interface 94 of the line card51 in processing a vectoring stream 125 will be described in more detailhereafter with particular reference to FIG. 6. The ring interfaces 94 ofthe other line cards 52 and 53 may be configured to operate similarly.

For illustrative purposes, assume that the control field 134 of thevectoring stream 125 has control bits respectively associated with theline cards 51-53 for indicating which of the line cards has failed, asdescribed above. Assume that all of the line cards 51-53 are initiallyoperational and, thus, have relay elements 91 in the pass-through state.

As shown by block 202 of FIG. 6, the ring interface 94 of the line card51 is configured to monitor the status of the line card 51 to detect acard failure. If the ring interface 94 detects such a failure, the ringinterface 94 causes the relay element 91 of the line card 51 totransition to the bypass state, as shown by block 205 of FIG. 6. Notethat the relay element 91 may also automatically transition to thebypass state if power is removed from the relay element 91, such as mayoccur during a power source failure.

As shown by block 208 of FIG. 6, the ring interface 94 of the line card51 determines if it has newly received a vectoring field 131-133 fromthe ring connection 80. If not, the ring interface 94 determines if ithas newly received the control field 134 from the ring connection 80, asshown by block 212 of FIG. 6. If so, the ring interface 94 toggles thecontrol bit of the field 134 associated with the line card 51, as shownby block 216 of FIG. 6, thereby indicating that the line card 51 isoperational. As shown by block 219 of FIG. 6, the ring interface 94 alsochecks the other control bits of the control field 134 to determinewhether any of such control bits have not been toggled since the lasttime that the control field 134 was transmitted from the line card 51.If any such bit has not been toggled, the ring interface 94 determinesthat the line card 52 or 53 associated with the non-toggled bit hasfailed and stores data indicative of such failure, as shown by block 222of FIG. 6. As will be described in more detail below, such data will beused to determine when to zero a vectoring field allocated to the failedcard.

Upon newly receiving a vectoring field 131-133, the ring interface 94determines whether the received vectoring field is allocated to the linecard 51, as shown by block 234. If so, the ring interface 94 overwritesthe vectoring field 131 with new vectoring information from the card'svector logic 77 and transmits the field 131 to the next line card 52 viathe ring connection 80, as shown by blocks 235 and 238 of FIG. 6.Specifically, the ring interface 94 inserts into the time slots of thevectoring field 131 tone symbols that are to be transmitted or have beenreceived by the transceivers 55 of the line card 51.

If the received vectoring field is not allocated to the line card 51,then the ring interface 94 determines, as shown by block 244 of FIG. 6,whether the line card 52 or 53 to which the vectoring field is allocatedhas failed based on the data stored via block 222. If such card 52 or 53has failed, then the ring interface 94 of the line card 51 zeros thevectoring field by overwriting it with all zeros, as shown by block 249of FIG. 6. If such card 52 or 53 has not failed, then the ring interface94 of the line card 51 passes the vectoring information from thereceived vectoring field to the vector logic 77 of the line card 51, asshown by block 652 of FIG. 6, so that the vector logic 77 can performcrosstalk vectoring based on such vectoring information. After eitherzeroing the received vectoring field in block 249 or passing thevectoring information from the received vectoring field to the vectorlogic 77 in block 252, the ring interface 94 of the line card 51transmits the vectoring field to the next line card 52 via the ringconnection 80, as shown by block 238.

By implementing a similar process at each line card 51-53, vectoringinformation is passed from card-to-card around the ring formed by thering connection 80 and cards 51-53. If a line card fails, its respectiverelay element 91 is transitioned to the bypass state such that thevectoring information of the operational cards continues to becommunicated among the operational cards bypassing the failed card.Thus, the card failure does not prevent the operational cards fromreceiving vectoring information for performing crosstalk vectoring.

It should be emphasized that the configurations and techniques describedherein for handling card failures in a multi-card vectoring group areexemplary, and various changes and modifications to the disclosedembodiments would be apparent to a person of ordinary skill in the art.As an example, as will be described in more detail below, a card failuremay be handled by transmitting vectoring information in oppositedirections around the ring. In yet other embodiments, other techniquesfor handling card failures are possible.

FIG. 7 depicts an exemplary embodiment of six line cards 301-306 coupledvia a ring connection 311 similar to the line cards 51-53 and ringconnection 80 of FIG. 2, though other numbers of line cards 301-306 maybe implemented in other embodiments. Such line cards 301-306 may beimplemented at the network access point 25 (FIG. 1) in addition to or inlieu of the cards 51-53 shown by FIG. 2. The ring connection 311comprises a plurality of segments 321-326 that are coupled between theline cards 301-306, as shown. Each line card 301-306 may be configuredlike the line cards 51-53 shown by FIG. 2, though other configurationsof the line cards 301-306 are possible in other embodiments. Thus, asdescribed above for the line cards 51-53 of FIG. 2, the line cards301-306 are configured to transmit a vectoring stream via the ringconnection 311. Further, on a network side, each line card 301-306 maybe coupled to the switching element 71 (FIG. 2), like the line cards51-53.

When a line card 301-306 fails, the other operational line cards areconfigured to bypass the failed card with the vectoring streamcommunicated across the ring connection 311. For example, the line cards301-306 may have relay elements 91, like the line cards 51-53, such thatthe failed card is bypassed by transitioning its respective relayelement 91 to the bypass state. However, in another exemplaryembodiment, the line cards 301-306 do not have a relay element 91, andthe failed card is bypassed via other techniques. For example, as willbe described in more detail hereafter, the failed line card can bebypassed by transmitting the vectoring stream in opposite directionsaround the ring.

FIG. 8 depicts an exemplary embodiment of a line card 301, which doesnot have a relay element 91. Except for the missing relay element 91,the exemplary line card 301 of FIG. 8 has the same components as theline cards 51-53. As shown by FIG. 8, the ring interface 94 is coupleddirectly to the ring ports 87 and 88. The other line cards 302-306 maybe configured like the exemplary line card 301 shown by FIG. 8.

FIG. 9 depicts an exemplary data flow for the line cards 301-306. In theexemplary embodiment shown by FIG. 9, the vectoring stream istransmitted in the counterclockwise direction around the ring, thoughthe vectoring stream may be normally transmitted in the clockwisedirection around the ring in other embodiments. When a card failureoccurs, the failure is detected by another line card, referred tohereafter as the “protect card,” that is immediately prior to the failedline card (i.e., that transmits the vectoring stream directly to thefailed line card). When the protect card receives data from the ringconnection 311, the protect card transmits the data in the oppositedirection away from the failed card. Thus, the vectoring stream 125 iscommunicated in both directions around the ring so that each operationalline card receives the vectoring stream.

Note that there are various techniques that can be used to communicatedata in both directions. In this regard, a different physical medium canbe used to transmit the vectoring stream in one direction relative tothe other direction. In such case, there are essentially two ringconnections. One of the ring connections carries data in one directionwhile the other ring connection carries data in the opposite direction.In another embodiment, the same segments 321-326 (FIG. 7) can be used tocarry data in both directions. In such an embodiment, the data in onedirection may be transmitted at a different frequency (frequencydivision multiplexing) or at different times (time divisionmultiplexing) so that the data flow in one direction does not interferewith the data flow in the opposite direction.

To better illustrate the operation of the line cards 301-306, assumethat the vectoring stream is being transmitted counterclockwise, asshown by FIG. 9, when the line card 302 fails. Upon such failure, theline card 303 stops receiving the vectoring stream from the line card302. After a predefined amount of time has passed without receiving datafrom the line card 302, the ring interface 94 (FIG. 8) of the line card303 determines that the card 302 has failed. In response, such ringinterface 94 communicates a failure notification to the other line cards301 and 304-306 informing such other line cards of the failure. As anexample, the ring interface 94 of the line card 303 may be configured totransmit the failure notification via a control field of the vectoringstream. In other embodiments, other techniques for discovering and/orcommunicating the failure of the line card 302 are possible.

Upon learning of the failure of the next line card 302 in the ring, thering interface 94 (FIG. 8) of the line card 301 immediately prior to thefailed line card 302 begins to transmit the vectoring stream in theopposite direction around the ring such that the vectoring stream istransmitted around the ring in the clockwise direction, as shown by FIG.10. Further, the vectoring field allocated to the failed line card 302is zeroed, as described above, so that the operational cards 301 and303-306 do not modify tone symbols in an attempt to cancel crosstalkassociated with the failed line card 302. Zeroing of the vectoring fieldallocated to the failed line card 302 may be performed by the ringinterface 94 of the line card 301 or any of the other operational linecards 303-306.

Note that the line card 303 that is immediately after the failed linecard 302 continues to transmit the vectoring stream in the samedirection as before the card failure. Thus, when the line card 303receives from the line card 304 a vectoring field allocated to anotheroperational line card 301 or 304-306, the ring interface 94 (FIG. 8) ofthe line card 303 transmits the vectoring field to the next line card304 as it did prior to the failure of the line card 302. Accordingly,the vectoring stream 125 travels in both directions among theoperational cards 301 and 303-306 effectively bypassing the failed linecard 302.

Further, as described above for the embodiment depicted by FIG. 2, whena given line card 301 or 303-306 receives the vectoring field allocatedto it, the card overwrites such vectoring field with new vectoringinformation. However, each of the line cards 304-306 between the twoline cards 301 and 303 that are adjacent to the failed line card 302 inthe ring preferably changes the vectoring field allocated to it for onlyone direction of the vectoring stream 125 so that it can be ensured thateach operational line card 301 and 303-306 has access to each symbolcommunicated by the stream 125.

For example, in the embodiment depicted by FIG. 10, each line card304-306 may be configured to overwrite the vectoring field allocated toit only when the vectoring field is received in the clockwise direction.In this regard, assume that a vectoring field allocated to the line card304 is received from the line card 305. In such case, the line card 304does not overwrite the vectoring field but rather passes its allocatedvectoring field unchanged to the next line card 303. However, if avectoring field allocated to the line card 304 is received from the linecard 303, then the line card 304 overwrites the vectoring field with newvectoring information. Accordingly, the vectoring information insertedinto the vectoring stream 125 by the line card 304 should be received byall of the other operational line cards 301, 303, 305, and 306 before itis overwritten by the line card 304.

In the example shown by FIG. 10, the line cards 301 and 303 adjacent tothe failed line card 302 receive the vectoring stream in only onedirection. Thus, such line cards 301 and 303 are preferably configuredto overwrite their respective vectoring fields upon receipt. Forexample, the line card 303 is configured to update the vectoring fieldallocated to it when such vectoring field is received from the line card304, and the line card 301 is configured to update the vectoring fieldallocated to it when such vectoring field is received from the line card306.

In one exemplary embodiment, the line cards 301-306 may be configured totransmit the vectoring stream in both directions around the ring innormal operation. When a failure occurs, the cards 301-306 may thentransmit the vectoring stream as shown by FIG. 10 without losingvectoring information from such stream due to the card failure. Forexample, the line card 301 could be configured to detect the failure ofthe line card 302 when it stops receiving the vectoring stream from theline card 302. In such case, the line card 301 may begin operating asdescribed above relative to FIG. 10 without having to receive a failurenotification from the line card 303.

Once a line card failure is remedied, such as for example replacing thefailed line card, the line cards 301-306 preferably resume normaloperation and, thus, communicate across the ring and perform vectoringas described above prior to the card failure. As an example, a new linecard that replaces the previously failed line card may be configured totransmit via a control filed of the vectoring stream a notification thatthe line card is operational. In response to such notification, theother lines cards may be configured to begin communicating around thering, as described above prior to the failure. Various other techniquesfor resuming normal operation after remedying a card failure arepossible.

An exemplary operation and use of the ring interface 94 of the line card301 in processing a vectoring stream will be described in more detailhereafter with particular reference to FIG. 11. The ring interfaces 94of the other line cards 302-306 may be configured to operate similarly.

For illustrative purposes, assume that all of the line cards 301-306 areinitially operational and transmit a vectoring stream counterclockwise,as shown by FIG. 9. As shown by block 401 of FIG. 11, the ring interface94 of the line card 301 determines if it has newly received a vectoringfield from the previous line card 306 via the ring connection 311. Ifnot, the ring interface 94 determines if it has newly received a controlfield from the previous line card 306 via the ring connection 311, asshown by block 404 of FIG. 11. If so, the ring interface 94 processescontrol information within such field as may be appropriate, as shown byblock 407 of FIG. 11. After processing control information in block 407,the ring interface 94 of the line card 301 determines whether the nextline card 302 has failed, as shown by block 408 of FIG. 11. If not, thering interface 94 transmits the control field to the next line card 302,as shown by block 409 of FIG. 11. However, if the next line card 302 hasfailed, then the ring interface 94 of the line card 301 insteadtransmits the control field to the previous line card 306, as shown byblock 410 of FIG. 11.

If the ring interface 94 of the line card 301 determines in block 401that it has received a vectoring field, the ring interface 94 thendetermines whether the received vectoring field is allocated to the linecard 301, as shown by block 411. If so, the ring interface 94 overwritesthe vectoring field with new vectoring information from the vector logic77 of the line card 301, as shown by block 416 of FIG. 11. Specifically,the ring interface 94 inserts into the time slots of the vectoring fieldtone symbols that are to be transmitted or have been received by thetransceivers 55 of the line card 301.

If the received vectoring field is not allocated to the line card 301,then the ring interface 94 passes the vectoring information from thereceived vectoring field to the vector logic 77 of the line card 301, asshown by block 422 of FIG. 11, so that the vector logic 77 can performcrosstalk vectoring based on such vectoring information. Afteroverwriting the received vectoring field in block 416 or passing thevectoring information from the received vectoring field to the vectorlogic 77 in block 422, the ring interface 94 of the line card 301determines whether the next line card 302 has failed, as shown by block408 of FIG. 11. If not, the ring interface 94 transmits the vectoringfield to the next line card 302, as shown by block 409 of FIG. 11.However, if the next line card 302 has failed, then the ring interface94 of the line card 301 instead transmits the vectoring field to theprevious line card 306, as shown by block 410 of FIG. 11.

Accordingly, once the line card 302 fails, the line card 301 reversesthe direction in which it transmits the data from the connection 311such that the vectoring stream is transmitted in both directions aroundthe ring. As such, each operational line card 301 and 303-306 shouldreceive the vectoring stream so that crosstalk vectoring performed bythe operational cards 301 and 303-306 can continue despite the failureof the line card 302.

It should be emphasized that the embodiments described above areexemplary, and various changes and modifications to the describedembodiments would be apparent to a person of ordinary skill upon readingthis disclosure.

Now, therefore, the following is claimed:
 1. A network access point,comprising: a switching element; a ring connection; a first line cardcoupled to the switching element, the ring connection, and a firstplurality of subscriber lines, the first line card configured to cancelcrosstalk affecting first symbols communicated across the firstplurality of subscriber lines based on a vectoring stream carried by thering connection, thereby providing first crosstalk-compensated symbols,the first line card further configured to insert the first symbols intothe vectoring stream and to transmit the first crosstalk-compensatedsymbols to the switching element; a second line card coupled to theswitching element, the ring connection, and a second plurality ofsubscriber lines, the second line card configured to cancel crosstalkaffecting second symbols communicated across the second plurality ofsubscriber lines, thereby providing second crosstalk-compensatedsymbols, the second line card further configured to insert the secondsymbols into the vectoring stream and to transmit the secondcrosstalk-compensated symbols to the switching element, wherein thesecond line card is configured to receive the first symbols from thevectoring stream and to maintain a vectoring coefficient correspondingto a tone of one of the first symbols, wherein the second line card isconfigured to combine the one of the first symbols with the vectoringcoefficient, thereby providing an estimate of crosstalk induced by theone of the first symbols and interfering with one of the second symbols,and wherein the second line card is configured to combine the estimatewith the one of the second symbols, thereby cancelling the crosstalkinduced by the one of the first symbols; and a third line card coupledto the switching element, the ring connection, and a third plurality ofsubscriber lines, the third line card configured to cancel crosstalkaffecting third symbols communicated across the third plurality ofsubscriber lines, thereby providing third crosstalk-compensated symbols,the third line card further configured to insert the third symbols intothe vectoring stream and to transmit the third crosstalk-compensatedsymbols to the switching element, wherein the line cards are configuredto transmit the vectoring stream in a first direction around the ringconnection and in a second direction around the ring connection oppositeto the first direction thereby causing the vectoring stream to bypassthe first line card when a failure occurs on the first line card.
 2. Thenetwork access point of claim 1, wherein the second line card comprisesa ring interface coupled to the ring connection, and wherein the secondline card is configured to transmit the vectoring stream in the seconddirection in response to a detection of the failure.
 3. The networkaccess point of claim of claim 2, wherein the second line card isadjacent to the first line card in a ring formed by the ring connectionand the first, second, and third line cards.
 4. The network access pointof claim 1, wherein the second line card is adjacent to the first linecard in a ring formed by the ring connection and the first, second, andthird line cards, wherein the vectoring stream has time slots allocatedto the first line card, wherein the first line card is configured toinsert the first symbols into the time slots allocated to the first linecard, and wherein the second line card is configured to overwrite thetime slots allocated to the first line card.
 5. The network access pointof claim 1, wherein the switching element is configured to multiplex thefirst, second, and third crosstalk-compensated symbols thereby providinga multiplexed data stream for transmission to a network.
 6. A method foruse at a network access point, comprising: communicating first symbolsacross a first plurality of subscriber lines coupled to a first linecard; cancelling crosstalk affecting the first symbols based on avectoring stream carried by a ring connection connecting at least thefirst line card, a second line card, and a third line card, therebyproviding first crosstalk-compensated symbols; inserting the firstsymbols into the vectoring stream by the first line card; transmittingthe first cross-talk compensated symbols from the first line card to aswitching element at the network access point; communicating secondsymbols across a second plurality of subscriber lines coupled to thesecond line card; cancelling crosstalk affecting the second symbolsbased on the vectoring stream, thereby providing secondcrosstalk-compensated symbols; inserting the second symbols into thevectoring stream by the second line card; transmitting the secondcross-talk compensated symbols from the second line card to theswitching element; receiving the first symbols from the vectoring streamat the second line card; maintaining, at the second line card, avectoring coefficient corresponding to a tone of one of the firstsymbols; combining the one of the first symbols with the vectoringcoefficient, thereby providing an estimate of crosstalk induced by theone of the first symbols and interfering with one of the second symbols;combining the second estimate with the one of the second symbols,thereby cancelling the crosstalk induced by the one of the firstsymbols; communicating third symbols across a third plurality ofsubscriber lines coupled to the third line card; cancelling crosstalkaffecting the third symbols based on the vectoring stream, therebyproviding third crosstalk-compensated symbols; inserting the thirdsymbols into the vectoring stream by the third line card; transmittingthe third cross-talk compensated symbols from the third line card to theswitching element; transmitting the vectoring stream in a firstdirection around the ring connection; and transmitting the vectoringstream in a second direction around the ring connection opposite to thefirst direction thereby causing the vectoring stream to bypass the firstline card when a failure occurs on the first line card.
 7. The method ofclaim 6, further comprising detecting the failure, wherein thetransmitting the vectoring stream in the second direction is performedin response to the detecting.
 8. The method of claim 7, wherein thetransmitting the vectoring stream in the second direction is performedat the second line card, and wherein the second line card is adjacent tothe first line card in a ring formed by the ring connection and thefirst, second, and third line cards.
 9. The method of claim 6, whereinthe second line card is adjacent to the first line card in a ring formedby the ring connection and the first, second, and third line cards,wherein the method further comprises: allocating times slots of thevectoring stream to the first line card; inserting the first symbolsinto the time slots allocated to the first line card; and overwritingthe time slots allocated to the first line card at the second line card.10. The method of claim 6, wherein the vectoring stream has a firstplurality of time slots allocated to the first line card, a secondplurality of time slots allocated to the second line card, and a thirdplurality of time slots allocated to the third line card.
 11. The methodof claim 10, wherein the first line card is configured to insert thefirst symbols into the first plurality of time slots, wherein the secondline card is configured to insert the second symbols into the secondplurality of time slots, and wherein the third line card is configuredto insert the third symbols into the third plurality of time slots. 12.The method of claim 11, wherein the second line card is configured touse the first symbols inserted into the first plurality of time slotsfor cancelling crosstalk from the second symbols.